Internal combustion engines have been employed for over a century. However, with the continuing high cost of fuel and quest to reduce emissions, manufacturers have developed many methods to make the combustion process more efficient and, at the same time, with lower emissions. One of these methods is known as Exhaust Gas Recirculation (EGR). U.S. Pat. No. 3,872,845 describes an early EGR system as applied to a passenger vehicle. EGR has been shown extremely effective for removing NOx from internal combustion emissions.
EGR systems are also being employed in heavy duty diesel engines as pending environmental regulations are requiring significant reduction in NOx emissions. U.S. Pat. No. 4,055,158 teaches that an effective way to improve the EGR effectiveness in reducing NOx emission is to cool the recirculated exhaust before it is fed into the intake system. By cooling the exhaust from 400-800° F. to 150-250° F., the NOx emissions were reduced by an additional 10%. As an added advantage, cooling the EGR also allows lower charge air temperature (thus a higher air to fuel ratio) which is also critical to the efficiency of modem heavy diesel engines equipped with turbo charger and intercooler.
One problem associated with cooling the recirculated exhaust gasses is that the entrained moisture is likely to condense. Indeed, approximately 0.5 to 4% of the intake air is actually water vapor. In the exhaust gas, the water content is even higher due to the fact that water is one of the combustion byproducts. U.S. Pat. No. 4,055,158 teaches that a condensate trap should be situated downstream of the heat-exchange device. U.S. Pat. No. 6,367,256B1 further teaches that the condensate from the EGR stream is acidic in nature and causes engine component degradation. The '256 patent teaches that a series of condensate traps should be employed, the condensate later being reheated and discharged to the tailpipe as gasses. This solution reduces the environmental friendliness of the EGR system is by introducing gaseous acids to the air. At the same time it cannot remove the condensate formed after the traps, such as those in the intake manifold.
Another possible way to reduce the acidic gasses from the EGR stream is to remove NOx and SOx in the exhaust gas before recirculating to the engine intake. The techniques of using absorbents such as charcoal, calcium carbonate, and calcium hydroxide to remove harmful species such as carbon monoxide from the exhaust gas were developed many decades ago, as disclosed in U.S. Pat. Nos. 2,077,563 and 2,216,763. However, these techniques, without further refinements, have now been found to be less effective in general exhaust gas treatment compared to the modern catalysis/absorption techniques. U.S. Pat. No. 3,739,583 discloses the use of a catalyst bed in the exhaust stream to reduce the NOx level. The use of catalytic absorber to remove NOx, SOx, and CO in the exhaust gas has also been disclosed earlier in U.S. Pat. No. 5,451,558. In these catalytic absorber systems, the catalysts and absorbers are either fabricated together or located in proximity. The catalysts oxidize the NO, SO2 and CO to NO2, SO3, and CO2, respectively, so that they can be readily absorbed by the absorbers.
Although treating the entire exhaust gas is desirable for emission control, removing the acidic gases in the EGR stream is more critical to engine protection. A recent application WO 02/22239A1 teaches the use of a combined catalytic/sorber system that first oxidizes SO2 to SO3 and then removes the SO3 through an absorption process in an EGR stream.
The use of catalyst to remove NOx and SOx has been an important area of investigation in the last few years due to the future environmental regulations such as Euro IV and EPA 2007 emission standards. However, the technology for heavy duty diesel engines has not matured due to a number of issues around catalyst life such as SOx poisoning etc. In addition, most of the catalyst/sorber systems are based on a honeycomb structure and the diffusion rates of the NOx and SOx are critical to absorption efficiency.
One problem with the catalyst/sober systems is that they are highly sensitive to temperature. Indeed, EP 0597106B teaches that the sorber portion is not effective below 200° C. for the removal of NOx. Similarly, WO0222239A1 teaches that the sorber is far less effective below 400° C. for the removal of SOx. Another problem with a catalyst/sorber system is the significant heat generation of the system itself. Since diesel engines are more efficient with lower EGR stream temperature, the catalyst/sorber system process is inefficient because it adds heat to the EGR stream that must be removed.
Taking a different approach, GB2301865 teaches the use of an aqueous ammonia solution to treat carbon dioxide in an EGR stream. This technique is not suitable for heavy duty trucks since it requires the handling and recurring replenishment of large quantities of volatile ammonia and water.
The use of EGR has significant impact on the useful life of the engine lubricant because of the acids formed in the oil as oxidation products as well as the acidic nature of the blow-by gas, lubricating oils are formulated to be basic in nature. Indeed, “overbased” detergents are usually used to give the lubricant the capability to act as a neutralization agent. However, a recurring problem in EGR diesel engines is that the useful life of the lubricating oil is significantly reduced. Specifically, the basicity of the lubricating oil is rapidly depleted in this environment.